An array substrate of a display device includes a base substrate, and a plurality of gate lines and a plurality of data lines located the inside of the base substrate to intersect with each other to define a plurality of pixel units. In each of the pixel units, a TFT (thin film transistor) and a pixel electrode electrically connected with the TFT are provided.
Taking a bottom gate structure TFT as an example, the TFT in each of the pixel units includes a gate electrode provided on the base substrate, an active layer on the gate electrode, and a source electrode and a drain electrode respectively located at opposite sides of the active layer and both partially overlapped with the active layer. When a turn-on voltage is applied to the gate electrode, the active layer become conductive, so that electrons are transported from the source electrode to the drain electrode through the conducted active layer, so that holes are transported from the drain electrode to the source electrode through the conducted active layer. When a turn-off voltage is applied to the gate electrode, the active layer is cut off, so that the transportation of the electrons and holes between the source electrode and the drain electrode is interrupted.
In order to reduce contact resistance of the source electrode and the drain electrode with the active layer to improve the performance of the TFT, the TFT generally includes a first ohmic contact part located in the overlapping area between the source electrode and the active layer and a second ohmic contact part located at the overlapping area between the drain electrode and the active layer, and both are between the film layer on which the source electrode and the drain electrode are located and the active layer.
The material for the active layer is generally amorphous silicon semiconductor having relatively low conductivity, and therefore, after the turn-on voltage is applied to the gate electrode to make the active layer conductive, because the active layer has relatively low conductivity, the mobility of the electrons of the source electrode and the holes of the drain electrode is relatively low, and thus the on-state current of the TFT is relatively low, which causes the ratio of the on-state current to the off-state current of the TFT is relatively small, causing that the TFT cannot effectively drive a large area display device, for example, a large area OLED (organic light emitting diode).
Generally, in order to increase the on-state current of the TFT, the material for the active layer is generally a semiconductor material having relatively high conductivity, such as doped semiconductor, crystalized semiconductor or metal oxide semiconductor, or the like. However, because the conductivity of the material of the active layer is increased, although the on-state current of the TFT is increased, the off-state current of the TFT is also increased, resulting in the ratio of the on-state current to the off-state current of the TFT still low. Furthermore, the TFT cannot still effectively drive a large area display device.